Not for some time has there been as significant a developement in telecommunications as lightwave technology which is manifested in the use of lightguide fibers. Optical or lightguide fibers are versatile as a transmission medium--all forms of information be it voice, video, or data can be carried on a lightguide fiber. Also, lightwave systems are ideally suited to the high bandwidth requirement of digital transmission and hence are well-matched to the evolving transmission network in this country.
Fibers for lightwave communications are drawn from a preform--an elongated cylinder of glass having an inner core and an outer cladding--with the thickness of the core and the cladding being in the same ratio in the fiber as they are in the preform. A drawing system is well-described in an article by D. H. Smithgall and D. L. Myers in the Winter 1980 issue of THE WESTERN ELECTRIC ENGINEER. The preform which is 7 to 25 mm in diameter and as much as 100 cm in length is heated locally and symmetrically to temperatures in excess of 2000.degree. C. As the preform is fed into a heating zone, such as in a furnace for example, fiber is drawn from molten material of a necked-down portion, yielding a substantial replica of the preform cross-section.
The drawn fiber is coated and taken up on a lightweight, plastic spool such that end portions of the fiber on each spool are available for testing. The spools of drawn, tested fiber are subsequently used to supply ribbon and/or cabling apparatus.
During the fiber drawing process, contaminants such as dust can pass from the heating zone or ambient air to the glass preform, its molten portion or the drawn fiber. As a result, the tensil strength of the fiber is reduced considerably and attenuation may be increased. A discussion of these problems is contained in an article by H. Aulich et al "Preparation of Optical Fibers of High Tensile Strength," Siemens Forschungs-Und Entwicklungsperreicht, Volume 7, No. 3, 1978, pps. 165-168.
Production of long lengths of fibers with high strength is crucial to increasing fiber yields and lowering the cost. Although long lengths of relatively high strength fiber have been produced in a laboratory atmosphere, their routine manufacture in a production environment has remained a challenge. The factors affecting fiber strength are well understood.
The causes of low strength fiber fall into several different categories. These include surface damage due to foreign particles at the fiber surface, surface scratching due to abrasion with a foreign surface, surface damage on the thin side of the coating of a poorly centered fiber, and internal voids or inclusions in the glass.
It is well known that surface flaws are the major causes of strength reduction. There are numerous materials and processing parameters that affect the severity and distribution of the surface flaws. One of the major factors contributing to the generation of surface flaws in a drawing environment is the accumulation of particles on the glass surface of the drawn fiber. Inorganic particles in and above the furnace or in the environment between the furnace and a coating applicator can impact the fiber surface to produce scratches, or can be drawn inside the furnace by the upward draft and impact the necked-down portion of the preform thereby causing weak points. Also, particles can descend into the coating applicator and engage the surface of the drawn fiber passing through the coating.
It should be readily apparent that one obvious solution to the problem of surface contamination is to cause the entire room in which a plurality of drawing apparatus are located to have a predetermined cleanliness. This kind of arrangement, which requires a substantial investment of capital is well known to the industry that manufactures electronic devices such as semiconductors. There, the intent is to provide a totally clean environment. In contrast, in drawing lightguide fiber, the intent is to keep the glass clean during the drawing operation which seemingly generates more contaminants than do operations in the electronic devices industry. Prior to the drawing operation, the preform is stored in clean chambers, and after the fiber is drawn, it is coated to protect it from atmospheric contaminants.
What is needed and what the prior art seemingly does not provide are methods and apparatus for causing the drawing of lightguide fiber from a preform to be accomplished in a clean environment. A clean air delivery and maintenance system is required to prevent contaminants from settling on the preform and the glass fiber in order to avoid the degradation of the strength of the drawn fiber. If at all possible, a solution to this problem should not be capital intensive.